Noninvasive Stimulation of Neurotypic Cells Using Persistent Photoconductivity of Gallium Nitride

Effects of surface properties of GaN semiconductors on cell behavior

In recent years, semiconductors have aroused great interest in connecting, observing and influencing the behavior of biological elements, and it is possible to use semiconductor-cell compound interfaces to discover new signal transduction in the biological field. Among them, III-V nitride semiconductors, represented by gallium nitride (GaN), are used as substrates to form semiconductor-biology interfaces with cells, providing a platform for studying the effects of semiconductors on cell behavior. The interfaces between GaN substrate and cells play an important role in detecting and manipulating cell behaviors and provide a new opportunity for studying cell behavior and developing diagnostic systems. Hence, it is necessary to understand how the properties of the GaN substrate directly influence the behavior of biological tissues, and to create editable biological interfaces according to the needs. This paper reviews the synergism between GaN semiconductors and biological cells. The electrical properties, persistent photoconductivity (PPC), nanostructures, and chemical functionalization of GaN on the promotion of cell behaviors, such as growth, adhesion, differentiation, and signal transduction, are emphatically introduced. The purpose of this study is to provide guidance to explore the detection and regulation methods of cell behavior based on semiconductors and promote the application of them in the field of bioelectronics, such as biochips, biosensors, and implantable systems.

Tuning Microbial Activity via Programmatic Alteration of Cell/Substrate Interfaces

A wide portfolio of advanced programmable materials and structures has been developed for biological applications in the last two decades. Particularly, due to their unique properties, semiconducting materials have been utilized in areas of biocomputing, implantable electronics, and healthcare. As a new concept of such programmable material design, biointerfaces based on inorganic semiconducting materials as substrates introduce unconventional paths for bioinformatics and biosensing. In particular, understanding how the properties of a substrate can alter microbial biofilm behavior enables researchers to better characterize and thus create programmable biointerfaces with necessary characteristics on demand. Herein, the current status of advanced microorganism-inorganic biointerfaces is summarized along with types of responses that can be observed in such hybrid systems. This work identifies promising inorganic material types along with target microorganisms that will be critical for future research on programmable biointerfacial structures.

Behavior of E. coli with Variable Surface Morphology Changes on Charged Semiconductor Interfaces

Bacterial behavior is often controlled by structural and composition elements of their cell wall. Using genetic mutant strains that change specific aspects of their surface structure, we modified bacterial behavior in response to semiconductor surfaces. We monitored the adhesion, membrane potential, and catalase activity of the Gram-negative bacterium Escherichia coli (E. coli) that were mutant for genes encoding components of their surface architecture, specifically flagella, fimbriae, curli, and components of the lipopolysaccharide membrane, while on gallium nitride (GaN) surfaces with different surface potentials. The bacteria and the semiconductor surface properties were recorded prior to the biofilm studies. The data from the materials and bioassays characterization supports the notion that alteration of the surface structure of the E. coli bacterium resulted in changes to bacterium behavior on the GaN medium. Loss of specific surface structure on the E. coli bacterium reduced its sensitivity to the semiconductor interfaces, while other mutations increase bacterial adhesion when compared to the wild-type control E. coli bacteria. These results demonstrate that bacterial behavior and responses to GaN semiconductor materials can be controlled genetically and can be utilized to tune the fate of living bacteria on GaN surfaces.

Modification of the Surface Properties of Al x Ga1-x N Substrates with Gradient Aluminum Composition Using Wet Chemical Treatments

The surface properties of biomolecular gradients are widely known to be important for controlling cell dynamics, but there is a lack of platforms for studying them in vitro using inorganic materials. The changes in various surface properties of an Al x Ga1-x N film (0.173 ≤ x ≤ 0.220) with gradient aluminum content were quantified to demonstrate the ability to modify interfacial characteristics. Four wet chemical treatments were used to modify the surface of the film: (i) oxide passivation by hydrogen peroxide, (ii) two-step functionalization with a carboxylic acid following hydrogen peroxide pretreatment, (iii) phosphoric acid etch, and (iv) in situ functionalization with a phosphonic acid in phosphoric acid. The characterization confirmed changes in the topography, nanostructures, and hydrophobicity after chemical treatment. Additionally, X-ray photoelectron spectroscopy was used to confirm that the chemical composition of the surfaces, in particular, Ga2O3 and Al2O3 content, was dependent on both the chemical treatment and the Al content of the gradient. Spectroscopic evaluation showed red shifts in strain-sensitive Raman peaks as the Al content gradually increased, but the same peaks blue-shifted after chemical treatment. Kelvin probe force microscopy measurements demonstrated that one can modify the surface charge using the chemical treatments. There were no predictable or controllable surface charge trends because of the spontaneous oxide-based nanostructured formations of the bulk material that varied based on treatment and were defect-dependent. The reported methodology and characterization can be utilized in future interfacial studies that rely on water-based wet chemical functionalization of inorganic materials.

Giant UV Photoresponse of GaN-Based Photodetectors by Surface Modification Using Phenol-Functionalized Porphyrin Organic Molecules

Organic molecular monolayers (MoLs) have been used for improving the performance of various electronic device structures. In this work, the concept of organic molecular surface modification is applied for improving the performance of GaN-based metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs). Organic molecules of phenol-functionalized metallated porphyrin (hydroxyl-phenyl-zinc-tetra-phenyl-porphyrin (Zn-TPPOH)) were adsorbed on GaN, and Ni/Zn-TPPOH/GaN/Zn-TPPOH/Ni PD structures were fabricated. This process was beneficial in two ways: first, the reverse-bias dark current was reduced by 1000 times, and second, the photocurrent was enhanced by ∼100 times, in comparison to the dark and photocurrent values obtained for Ni/GaN/Ni MSM PDs, at high voltages of ±10 V. The responsivity of the devices was increased from 0.22 to 4.14 kA/W at 5 μW/cm² optical power density at -10 V bias and at other voltages also. In addition to this, other PD parameters such as photo-to-dark current ratio and UV-to-visible rejection ratio were also enhanced. The spectral selectivity of the PDs was improved, which means that the molecularly modified devices became more responsive to UV spectral region and less responsive to visible spectral region, in comparison to bare GaN-based devices. Photoluminescence measurements, power-dependent photocurrent characteristics, and time-resolved photocurrent measurements revealed that the MoL was passivating the defect-related states on GaN. In addition, Kelvin probe force microscopy showed that the MoL was also playing with the surface charge (due to surface states) on GaN, leading to increased Schottky barrier height in dark conditions. Resultant to both these phenomena, the reverse-bias dark current was reduced for metal/MoL/GaN/MoL/metal PD structures. Further, the unusual photoconductive gain in the molecularly modified devices has been attributed to Schottky barrier lowering for UV-illuminated conditions, leading to enhanced photocurrent.

Highly Stable Persistent Photoconductivity with Suspended Graphene Nanoribbons

Graphene nanoribbon (GNR), also known as 1-dimensional graphene, with a non-zero band gap has a huge potential for various electrical and optoelectrical applications because of its high transparency, flexibility, controllable band gap, and unique edge states. Recent advances in the synthesis of GNR enable us to show the possibility of GNRs as future high performance electrical devices. However, the applicability of GNRs to optoelectrical devices is unclear. Here we report that suspended GNR devices can show persistent photoconductivity (PPC) with long decay time (over 72 h) and adequate environmental stability. Repeated non-volatile memory operation is also demonstrated with an integrated PPC device using GNRs. This very stable PPC device can be applied to a wide variety of fields such as ultra-low-power non-volatile memory, nanoscale imaging, and biological sensors. Our results have opened the door to advance the study of GNRs in novel directions such as optoelectrical applications.

Bioelectronics communication: encoding yeast regulatory responses using nanostructured gallium nitride thin films

Baker's yeast, S. cerevisiae, is a model organism that is used in synthetic biology. The work demonstrates how GaN nanostructured thin films can encode physiological responses in S. cerevisiae yeast. The Ga-polar, n-type, GaN thin films are characterized via Photocurrent Measurements, Atomic Force Microscopy and Kelvin Probe Force Microscopy. UV light is used to induce persistent photoconductivity that results in charge accumulation on the surface. The morphological, chemical and electronic properties of the nanostructured films are utilized to activate the cell wall integrity pathway and alter the amount of chitin produced by the yeast. The encoded cell responses are induced by the semiconductor interfacial properties associated with nanoscale topography and the accumulation of charge on the surface that promotes the build-up of oxygen species and in turn cause a hyperoxia related change in the yeast. The thin films can also alter the membrane voltage of yeast. The observed modulation of the membrane voltage of the yeast exposed to different GaN samples supports the notion that the semiconductor material can cause cell polarization. The results thus define a strategy for bioelectronics communication where the roughness, surface chemistry and charge of the wide band gap semiconductor's thin film surface initiate the encoding of the yeast response.

Ga Ion-Enhanced and Particle Shape-Dependent Generation of Reactive Oxygen Species in X-ray-Irradiated Composites

The reported results test the effects of the collective behavior hypothesized to contribute to the production of more reactive oxygen species (ROS) in vitro and result in an enhanced radiosensitization. The role of particle shape in composites with gallium oxyhydroxide (GaOOH) particles and Matrigel is studied. Particles of two different shapes are embedded into the gel to understand only the materials effect on the generation of ROS rather than cell penetrating variations. The paper reports materials characterization by scanning electron microscopy and X-ray diffraction. The stability of the particles within the composite is assessed by quantification of leached metal using inductively coupled plasma mass spectrometry. The amount of ROS in each construct under variable radiation conditions is quantified in the presence and absence of PC12 cells seeded on top of the composites. The viability of cells is also recorded under different in vitro conditions. The collective materials characterization and the results from the bioassays are used to explain the role of anisotropy on the radiosensitization of nanostructures containing Ga. The presence of Ga ions in composites can have a radiosensitizing effect, and the amount of the available Ga³⁺ determines the magnitude of the radiosensitization. The shape of the particles determines the stability in aqueous solutions and release of Ga³⁺ that triggers ROS production. The concentration and shape of Ga-containing materials can be combined to generate an additive effect by increasing the amount of available free metal ions in solution. The studies with GaOOH containing composites enable one to explore the role of key parameters that lead to an increased efficiency of radiation treatments.

Variably doped nanostructured gallium nitride surfaces can serve as biointerfaces for neurotypic PC12 cells and alter their behavior

Neurotypic PC12 cells behavior was studied on nanostructured GaN and rationalized with respect to surface charge, doping level, and chemical functionalization. The semiconductor analysis included atomic force microscopy, Kelvin probe force microscopy, and X-ray photoelectron spectroscopy. The semiconductor surfaces were then evaluated as biointerfaces, and the in vitro cell behavior was quantified based on cell viability, reactive oxygen species production, as well as time dependent intracellular Ca concentration, [Ca²⁺]_i, a known cell-signaling molecule. In this work, we show that persistent photoconductivity (PPC) can be used to alter the surface properties prior to chemical functionalization, the concentration of dopants can have some effect on cellular behavior, and that chemical functionalization changes the surface potential before and after exposure to UV light. Finally, we describe some competing mechanisms of PPC-induced [Ca²⁺]_i changes, and how researchers looking to control cell behavior non-invasively can consider PPC as a useful control knob.

Neurotypic PC12 cells behavior was studied on nanostructured GaN and rationalized with respect to surface charge, doping level, and chemical functionalization.